Do most 20 year old men cheat and do they really want to seattle down with their current girlfriends for the rest of there life?

I am going to split your question in two due to its self-conflicting nature.
In answer to the first question, "Do most 20 year old men cheat?", there is no answer because it is dependent on the person. Any man (or woman), from the ages of 20 to 80, will always ask themselves the question of "what if?" Since curiosity is not a fallacy in human logic, you should not be mad over this. If, however, this curiosity develops in the form of a man (or woman) cheating, then there is a problem, namely the inconsiderate nature of the relationship. If one person feels such a strong need to answer the question of, "what if," then they should not do so within the realm of a monogamous relationship.

In answer to the second question, it is once again dependent on the person. Majority of the relationships and marriages started from the ages of 18 to 23 will end within five years of their initiation. It is not unreasonable for a man to wish to stay with their current girlfriend because of the comfort, reliability, or pattern they like within their relationship dynamic, but do realize that men are not always content with their current arrangement, always striving for change or "improvement", so a mistake/action is bound to take place on their side.